BUTTERFLY sex is not as elegant an affair as you might think. It seems that male monarch butterflies conduct an all-out sperm war based on a crude measure of how much sperm is stored inside a female from a previous mating.
During sex the males physically restrain the females for an entire day while injecting them with a fluid which contains fertile sperm as well as seemingly functionless cells without nuclei.

Michelle Solensky of The College of Wooster in Ohio paired male monarch butterflies with a selection of females that had had different numbers of partners.

She found that males could selectively increase or decrease the amount of fertile sperm in their deposits. For example, they deposited slightly more into a female for each of her previous mates (Animal Behaviour, DOI: 10.1016/j.anbehav.2008.10.026). “This may explain earlier observations that the last male to mate has a reproductive advantage,” says Solenksy.
She then arranged for some female butterflies to receive a large deposit from a single male, and others to have a small deposit from three different males adding up to a similar volume.

When males later mated with the females, they used the same amount of sperm irrespective of which experimental group the female butterfly had been in. This showed that the males were adjusting their sperm on the basis of volume – not the number of previous partners.

“I don’t know of any other creatures that respond to the amount of sperm inside their mates,” says Solensky. “The new aspect for butterflies is that they can assess the intensity of sperm competition without ever witnessing previous matings,” says Simone Immler at the University of Sussex in the UK.

Because monarch butterflies do not use chemical signals like pheromones, Solensky suspects that sensors on the male penis detect the volume directly, like the dipstick in a car’s oil tank. If so, the cells that lack nuclei may act to bump up the volume of the deposit and discourage rivals.

This behaviour backs a theory that males of some species can boost their sperm levels to raise the odds of passing on their genes. Male fish, for example, release more sperm into the water when they sense a nearby rival.

Even men who spend more time bonding with their girlfriend unconsciously release more sperm during sex. “Males can be just as choosy as females; sperm may be cheaper to make than an egg, but it still isn’t free,” says Solensky.